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3-1982

Seasat-Satellite Investigation of the Structure ofWestern Nebraska and Its Application to theEvaluation of Geothermal ResourcesJohn Stix

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Stix, John, "Seasat-Satellite Investigation of the Structure of Western Nebraska and Its Application to the Evaluation of GeothermalResources" (1982). US Department of Energy Publications. 27.http://digitalcommons.unl.edu/usdoepub/27

LA-9275-H D R

UC-66b Issued: March 1982

LA--9275-HDR

DE82 014128

Seasa t-Satel Ii te Investigation of the Structure of Western Nebraska and Its

Application to the Evaluation of Geothermal Resources

n n n

Los Alamos National Laboratory Los Alamos,New Mexico 87545

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

SEASAT SATELLITE I N V E S T I G A T I O N OF THE STRUCTURE OF WESTERN NEBRASKA AND I T S APPLICATION TO THE EVALUATION-OF GEOTHERMAL RESOURCES

John S t i x

ABSTRACT

Seasa t s y n t h e t i c a p e r t u r e r a d a r ( S A R ) s a t e l l i t e imagery was used t o i n t e r p r e t t h e s t r u c t u r a l framework. and, i n d i r e c t l y , t h e geothermal p o t e n t i a l o f an area i n western Nebraska. Lineaments were mapped f rom t h e imagery and then compared t o known s t r u c t u r e . It was found t h a t Seasat does r e c o r d s u r f a c e m a n i f e s t a t i o n s o f s u b t l e basement s t r u c t u r e s , p a r t i c u l a r l y f a u l t s and j o i n t s . Fur thermore, two areas w i t h h o t d r y rock geothermal p o t e n t i a l were de l i n e a t e d u s i n g Seasat and o t h e r data. It i s s t ressed t h a t more subsur face geo logy and geophysica l da ta a re needed b e f o r e a f i n a l eva l - u a t i o n o f t h e geothermal p o t e n t i a l can be made. Seasat ima- ge ry i s a u s e f u l reconnaissance e x p l o r a t i o n t o o l i n t h e i n t e r p r e t a t i o n o f r e g i o n a l s t r u c t u r e w i t h i n areas o f l i t t l e t opograph ic re1 i e f .

I. INTRODUCTION

The purpose of us ing Seasat s y n t h e t i c a p e r t u r e rada r (SAR) s a t e l l i t e ima- gery i n western Nebraska was t o see i f s u r f a c e exp ress ion o f s u b t l e s t r u c t u r a l f e a t u r e s i n an area o f l i t t l e r e l i e f c o u l d be de tec ted u s i n g t h i s remote sens ing techn ique. Pr imary o b j e c t i v e s o f t h i s s tudy were t o examine f o l d s , f a u l t s , f r a c t u r e s , and j o i n t s u s i n g t h i s method; i f these s t r u c t u r e s c o u l d be de tec ted , a f u r t h e r examinat ion t r i e d t o separa te s t r u c t u r a l forms. Such a reconnaissance i s e s p e c i a l l y p romis ing i n t h e c o n t e x t o f a l a r q e r e x p l o r a t i o n program; it i s a way t o save t i m e and money i n p i n p o i n t i n g s p e c i f i c areas f o r more d e t a i l e d subsur face g e o l o g i c a l and geophys ica l e x p l o r a t i o n .

The area i n q u e s t i o n i s i n t e r e s t i n g f rom a geothermal s t a n d p o i n t , as t h e r e i s a geothermal anomaly, based on s i 1 i c a and sodium-potass ium-calc ium (Na-K-Ca) geothermometry, c o v e r i n g much o f western Nebraska (Swanberg and Morgan 1978; M i d c o n t i n e n t Hot Dry Rock Study Group 1980). S ince geothermal anomalies appear t o be r e l a t e d t o t h e s t r u c t u r a l f e a t u r e s i n western Nebraska, Seasat imagery was used t o p i n p o i n t o t h e r areas o f geotherma p o t e n t i a l .

1

11. REGIONAL STRATIGRAPHY AND STRUCTURE OF WESTERN NEBRASKA

Precambrian basement rock t y p e s a long t h e Cambridge a r c h ( F i g . 1 ) a r e b e t t e r known than i n a d j a c e n t areas due t o t h e r e l a t i v e t h i n n i n g o f s e d i - mentary s e c t i o n ove r t h e a r c h and t h e g r e a t e r number o f o i l and gas d r i l l h o l e s t h a t have reached t h e Precambrian su r face . Much i n f o r m a t i o n i s s t i l l needed t o a c c u r a t e l y p o r t r a y basement l i t h o l o g y , b u t i t i s known t h a t t h e r e i s q u i t e a complex assemblage o f rocks. W i t h i n t h e area t h e d i s t r i b u t i o n o f g r a n i t e s , s c h i s t s , g r a n o f e l s , and gne i sses has been genera l l y d e l i n e a t e d ( P l a t e 1, i n s i d e back c o v e r ) ( L i d i a k 1 9 7 2 ) . R u b i d i u m - S t r o n t i u m a n d Potassium-Argon ages o f t h e s c h i s t s , g r a n o f e l s, and gne i sses suggest t h a t Pre- cambrian metamorphic rocks o f t h e Cambridge a r c h and a d j o i n i n g area a r e n o t younger t h a n 1800 Myr b.p., p r o b a b l y o f Precambrian X age ( L i d i a k 1972).

Merr iam and A t k i n s o n (1955) s t a t e d t h a t b i o t i t e g r a n i t e s have i n t r u d e d t h e o l d e r metamorphic rocks and a r e younger t h a n 1800 Myr b.p. L i d i a k (1972) and Z i e t z and o t h e r s (1966) have agreed w i t h t h i s t h e o r y by n o t i n g t h e c i r - c u l a r shapes, absence o f f o l i a t i o n , magnet ic h ighs , and n o r t h e a s t - t r e n d i n g l i t h o l o g i c anomal ies o f t h e s e g r a n i t i c bodies. L i d i a k (1969, 1972) has suggested t h r e e p e r i o d s o f g r a n i t e emplacement i n Nebraska; t h e e a r l i e s t o c c u r r e d 1700 Myr b.p. i n no r thwes te rn Nebraska, f rom 1570 t o 1580 Myr b.p. i n sou the rn Nebraska, and between 1510 and 1540 Myr b.p. i n n o r t h e r n Nebraska and sou the rn South Dakota. However, t h e ages o f t h e l a t t e r t w o p e r i o d s o f p l u t o n i s m may be o l d e r due t o l a t e r widespread thermal a c t i v i t y t h a t may have r e s e t r a d i o g e n i c c locks .

A1 ong t h e c r e s t o f t h e Chadron-Cambridge arch , sediment t h i c k n e s s e s range f rom 900 rn i n no r thwes te rn Nebraska t o 1300 m i n t h e w e s t - c e n t r a l r e g i o n t o 1250 m i n southwestern Nebraska. The a r c h system i s asymmetric; sediments t h i c k e n r a p i d l y west o f t h e Chadron arch , w h i l e t h e s e c t i o n t h i c k e n s more grad- u a l l y t o t h e eas t o f t h e a r c h ( W u l f 1964). I n c o n t r a s t , t h e o p p o s i t e occu rs i n t h e Cambridge a r c h area, w i t h t h e s e c t i o n t h i c k e n i n g more r a p i d l y t o t h e e a s t o f t h e a r c h than t o t h e west ( C a r l s o n 1966).

The sedimentary s e c t i o n on t h e f l a n k s o f t h e a r c h system i s c h a r a c t e r i z e d b y u n c o n f o r m i t i e s and t h i c k e n i n g o f Pennsylvanian, Permian, T r i a s s i c , and Cre- taceous rocks away f rom t h e a r c h axes ( K i n g 1951; Wulf 1964). Pre-Cenozoic r o c k u n i t s t h a t o v e r l i e t h e c r e s t o f t h e Chadron-Cambridge system a r e t h i n n e r t h a n t h o s e on t h e f l a n k s o f t h e a r c h due t o d e p o s i t i o n a l c o n t r o l by t h e a r c h and t o uncon fo rm i t i es caused by t h e e r o s i o n o f beds as t h e a r c h system was up1 i f t e d p e r i o d i c a l l y . F i g u r e 2 shows a g e n e r a l i z e d r e g i o n a l s t r a t i g r a p h y f o r wes te rn Nebraska.

I n Cambrian t h r o u g h M i s s i s s i p p i a n t i m e , d e p o s i t i o n was m in ima l . A h i g h l a n d area was c o i n c i d e n t a l w i t h t h e a r c h (Tab le I ) (Wu l f 1964). Much o f what was depos i ted up t h r o u g h M i s s i s s i p p i a n t i m e was removed a t t h e end o f t h a t p e r i o d d u r i n g broad u p l i f t o f t h e a r c h and e ros ion , r e s u l t i n g i n a widespread uncon fo rm i t y (La rson 1962). T ransg ress ion i n t h e Pennsylvanian t h o r o u g h l y b u r i e d t h e a r c h under mar ine sediments (Wul f 1964); t h i c k n e s s and d i s t r i b u t i o n o f f a c i e s was c o n t r o l l e d he re by t h e movement o f t h e a r c h .

The Cambridge a r c h was l e s s a c t i v e t h a n t h e Chadron a r c h d u r i n g t h e Permian, whereas d u r i n g E a r l y Cretaceous t i m e t h e o p p o s i t e was t h e case (Moore and Nelson 1974). Very l i t t l e T r i a s s i c and J u r a s s i c sed m e n t a t i o n occu r red on t h e a r c h system due t o c o n t i n u e d u p l i f t and e r o s i o n t h e r e (Larson 1962).

Post-Laramide T e r t i a r y sediments were unconformably d e p o s i t e d o v e r f o l d e d P a l e o z o i c and Mesozoic sedimentary rocks (La rson 1962; Wu f 1964).

2

EXPLANATION

Crest of Structural Arch

Locatio< of MOP

250 Miles 50 0 50 1 0 0 1 50 200 I--

50 0 50 100 150 200 250 Kilometers - - -

Fig. 1. Ma jo r t e c t o n i c elements o f t h e c e n t r a l C o r d i l l e r a n f o r e l a n d ( m o d i f i e d f r o m Bay ley and Muehl be rge r 1968).

3

Mire River

1

AN

k-1 SILURIAN

Shales, silk, or clays

Son&, sandstones

Gravel

Evaporites

Chalk

Limestone

a Dolomite n +;:: Precambrian basement

F ig . 2 . G e n e r a l i z e d P h a n e r o z o i c s t r a t i g r a p h y o f w e s t e r n N e b r a s k a ( m o d i f i e d f r o m B u r c h e t t 1969, and Moore and Nelson 1974, F i g u r e 1 ) .

4

TABLE I

PHANEROZOIC UPLIFT OF THE CAMBRIDGE ARCH

Data f rom Larson 1962 S c h i s l e r , A., i n d e p e n d e n t g e o l o g i s t , G o l d e n , C o l o r a d o , p e r s o n a l

S tan ley 1971 S t a n l e y and Wayne 1972 Wul f 1964

comnunicat ion (1980)

P e r i o d o f U p l i f t Evidence

E a r l y O r d o v i c i a n E r o s i o n a l u n c o n f o r m i t i e s

L a t e s t Devonian E r o s i o n a l u n c o n f o r m i t i e s

L a t e s t M i s s i s s i p p i a n E r o s i o n a l unconformi t i es

Pennsyl van ian T h i n n i n g o f sedimentary beds ove r u p l i f t

L a t e s t Permian t o T r i a s s i c Non-deposi t i on

E a r l y Cretaceous Fac ies changes ( t r a n s g r e s s i o n )

L a t e Cretaceous t o E a r l y E r o s i o n a l unconformi t i e s

01 igocene Warping o f beds

P l e i s t o c e n e Dra inage p a t t e r n s , f a c i e s changes

T e r t i a r y (Laramide)

A. T e c t o n i c Framework and H i s t o r y P l a t e 2 ( i n s i d e back cove r ) shows t h e known s t r u c t u r a l f e a t u r e s o f t h e

area. Note t h a t t h e s t r u c t u r a f framework i s o n l y g e n e r a l l y known as a lmost a l l of t h e i n f o r m a t i o n i s based on unpub l i shed o i l and gas w e l l da ta .

The most prominent Precambrian basement f e a t u r e i n western Nebraska i s t h e n o r t h w e s t - t r e n d i n g Chadron-Cambridge a r c h system (F ig . 1) , wh ich extends no r thwes t f rom t h e C e n t r a l Kansas U p l i f t t o t h e B lack H i l l s U p l i f t . The d i f - f e r e n t elements o f t h e a r c h system cannot , however, be cons ide red t o g e t h e r i n terms o f t e c t o n i c a c t i v i t y , as each p a r t o f t h e system has been a c t i v e a t d i f - f e r e n t t imes d u r i n g t h e Phanerozoic. R e g i o n a l l y , t h e r e has been a no r thwes t p r o g r e s s i o n o f t e c t o n i c a c t i v i t y w i t h t i m e (Merr iam and A t k i n s o n 1955). I t has been suggested t h a t t h e Cambridge a r c h i s r e l a t e d more c l o s e l y i n t i m e t o t h e n o r t h e a s t - t r e n d i n g Siouxana a r c h t h a n t o t h e Chadron a r c h o r C e n t r a l Kansas U p l i f t ( B a r r and B i e b e r 1961). U p l i f t o f t h e B lack H i l l s may have oc- c u r r e d l a t e r t h a n t h a t o f t h e Chadron arch ; l i k e w i s e , Cambridge a r c h t e c t o n i c a c t i v i t y came b e f o r e ' t h a t o f t h e Chadron, and movement on t h e C e n t r a l Kansas U p l i f t preceded Cambridge movement. I n i t i a l u p l i f t o f t h e Cambridge a r c h i s b e l i e v e d t o have occu r red d u r i n g P a l e o z o i c t ime , a l t h o u g h s i m i l a r t e c t o n i c a c t i v i t y on t h e Chadron a r c h and B lack H i l l s U p l i f t i s t hough t t o have occu r red d u r i n g t h e Mesozoic ( B a r r and B i e b e r 1961).

The Cambridge a r c h has been a c t i v e i n t e r m i t t e n t l y t h roughou t t h e Phanero- z o i c ; movements a r e summarized i n Tab le I. A d d i t i o n a l l y , S t a n l e y and Wayne (1972) have no ted r e c e n t movement on t h e arch, as ev idenced by a b r u p t changes i n r i v e r courses over t h e arch, d r a i n a g e p a t t e r n s , and antecedent streams i n

5

t h e reg ion . F u r t h e r , Woo l la rd (1958) has recorded ear thquakes a long t h e arch , and Farquhar (1957) has no ted a c t i v i t y on t h e system i n Kansas. He no ted t h i s i s p r i m a r i l y a v e r t i c a l movement r a t h e r than h o r i z o n t a l sho r ten ing . r e l a t i o n i s t r u e f o r t h e system as a whole, as these movements th rough have been epe i rogen ic r a t h e r than orogen ic . These obse rva t i ons o f a c t a long t h e a rch a re suppor ted by Adams' (1980) da ta demonst ra t ing presen t i l t i n g o f t h e m i d c o n t i n e n t reg ion .

I n summary, t h e r e c u r r e n t up1 i f t a l o n g t h e Cambridge-Chadron system g e s t s a fundamental zone o f weakness t h a t has e x i s t e d s i n c e l a t e M i d d l e Cambrian t i m e (Osterwa ld and Dean 1961).

Steep d i p s encountered d u r i n g d r i l l i n g show p o s s i b l e f a u l t s ( C u r t i s

t h a t T h i s t i m e v i t y -day

sug- Pre-

959: Wul f 1964; S c h i s l e r * ) . Sediments draped over an anomalously s teep Precambrian s u r f a c e s u g g e s t basement f a u l t i n g , as t h e c o n f i g u r a t i o n o f d i f f e r e n t sedimentary su r faces r e f 1 e c t s basement topography on s t r u c t u r e con tou r maps o f t h i s area. Thus, t h e r e i s a cor respond ing s teep d i p o f t h e sediments and a t h i c k e n i n g o f sedimentary s e c t i o n . It i s p o s s i b l e t h a t t h e sediments were depos i ted over Precambrian basement r i d g e s , caus ing t h e anomalously s teep d i p s on e i t h e r s ide ; f a u l t i n g would n o t be necessary i n t h i s case. To a l e s s e r e x t e n t , s teep d i p s a l s o may be due t o d i f f e r e n t i a l compaction o f sediments and r e c u r r e n t u p l i f t ( K i n g 1951). But such t h e o r i e s can be e l i m i n a t e d w i t h rega rd t o t h e mapped s teep d ips . Sedimentary isopachous maps do n o t show t h i c k e n i n g on t h e f l a n k s and t h i n n i n g i n t h e m i d d l e as would be expected w i t h such a d r a p i n g over basement knobs (Larson 1962). Larson (1962) a l s o has found g r e a t e r f r a c t u r i n g i n 1 imestones where s teep d i p occurs on s t r u c t u r e con tou r maps. Fur thermore, i t i s suggested t h a t t h e development o f f a u l t i n g occu r red s imu l taneous ly w i t h e p e i r o g e n i c movement on t h e a r c h (Larson 1962; Moore and Nelson 1974). Thus, u p l i f t o f t h e arch, e r o s i o n and r e s u l t i n g d e p o s i t i o n o f sediments o f f t h e arch, and basement f a u l t i n g a re e s s e n t i a l l y contemporaneous. F u r t h e r sedimentary d e p o s i t i o n can occur a s h o r t t i m e a f t e r t h e onset o f f a u l t i n g . A lso, i t i s i m p o r t a n t t o r e a l i z e t h a t t h e e p e i r o g e n i c movement on t h e Cambridge-Chadron system i s e s s e n t i a l l y v e r t i c a l . L i d i a k (1972) has no ted l i t t l e , i f any, Phanerozoic l a t e r a l s t r i k e - s l i p movement i n t h e reg ion , a l t h o u g h west-nor thwest and wes t - t rend ing Precambr ian c a t a c l a s t i c shear zones a r e prominent i n seve ra l areas ( P l a t e 1). T h i s f a c t i s c o r r o b o r a t e d by t h e s t y l e of f a u l t i n g , an ex t reme ly h i g h ang le d isp lacement ( S c h i s l e r * ) . Such h i g h ang le f a u l t i n g makes t h e v e r i f i c a t i o n o f s teep d i p as a m a n i f e s t a t i o n o f subsur face f a u l t i n g an ex t reme ly d i f f i c u l t t ask , as t h e r e i s no a d d i t i o n o r removal o f s e c t i o n r e c o g n i z a b l e f rom e l e c t r i c l o g d a t a on e i t h e r s i d e o f a f a u l t .

F u r t h e r ev idence of f a u l t i n g i s p rov ided by s teep g r a d i e n t s i n aeromag- n e t i c da ta i n p laces where Precambrian basement l i t h o l o g i c c o n t a c t s a r e n o t observed from d r i l l i n g reco rds ( Z i e t z and o t h e r s 1966); c o n t a c t s between Precambrian l i t h o l o g i c u n i t s a re f rom L i d i a k (1972) and shown i n P l a t e 1.

F a u l t s , s teep d i p s , and s teep aeromagnet ic g r a d i e n t s , shown on P l a t e 2, t r e n d g e n e r a l l y no r thwes t and p a r a l l e l t h e s t r i k e o f t h e Cambridge a r c h ( Z i e t z e t al. , 1966), i n d i c a t i n g t h a t t hese s t r u c t u r e s a r e r e l a t e d t o t h e a rch and were c r e a t e d d u r i n g epe i rogen ic up1 i f t . I n c o n t r a s t , few known s t r u c t u r e s t r e n d p a r a l l e l t o t h e Siouxana arch , suggest ing t h a t t h e Siouxana i s and was l e s s a c t i v e t e c t o n i c a l l y t han t h e Cambridge arch. A l t e r n a t i v e l y , t h e r e may be a l a c k o f da ta f o r t h i s area.

* S c h i s l e r , A., independent g e o l o g i s t , Golden, Colorado, persona l communication (1980) .

b

F o l d i n g i n t h e area, shown on P l a t e 2, e x h i b i t s two d i s t i n c t t rends . The f i r s t r o u g h l y p a r a l l e l s t h e Cambridge system. As an example, t h e f o l d s cen te red south o f N40" l a t i t u d e and WlOl"30' l o n g i t u d e show a change i n t r e n d d i r e c t i o n f rom s o u t h e r l y t o sou thwes te r l y , sugges t ing t h a t f o l d s a re a f f e c t e d by and f o l l o w t h e s t r i k e o f t h e arch. The f o l d i n g may have formed as b u c k l i n g and/or g r a v i t y s l i d i n g occur red w i t h p e r i o d i c u p l i f t o f t h e arch. A second d i s - t i n c t t r e n d o f f o l d s i s recogn ized cen te red about N40'45' l a t i t u d e , W10lo30' l o n g i t u d e . These a n t i c 1 i n e s and sync1 i n e s t r e n d eas t -west b u t i n v a r i a b l y p lunge t o t h e west. They a re o b l i q u e t o t h e a r c h system, i n d i c a t i n g t h a t t h e f o l d i n g formed by a d i f f e r e n t process. The number o f f o l d s i n t h i s t r e n d i n - creases t o t h e west -nor thwest o f f t h e s tudy area (Osterwa ld and Dean 1961). The t r e n d may be exp la ined i n l i g h t o f Permian e v a p o r i t e beds. T h i s c l u s t e r o f f o l d s corresponds w i t h t h i c k e v a p o r i t e d e p o s i t s o f Upper Leonard ian age i n western Nebraska and a 75-m-thick s e c t i o n o f Wolfcampian age e v a p o r i t e s i n southwestern Nebraska (Rascoe and Baars 1972) . Thus, s a l t - t e c t o n i c s may be p a r t l y r e s p o n s i b l e f o r de fo rma t ion o f beds i n t o broad, g e n t l e f o l d s a long t h e f l a n k s o f t h e arch. The e v a p o r i t e d e p o s i t s a r e i r r e g u l a r i n e x t e n t , w i t h ab rup t f a c i e s changes t o c l a s t i c rocks i n t h e eas t i n t h e area of t h e Cambridge arch. However, i t i s p o s s i b l e t h a t a c t i v i t y on t h e Cambridge system cou ld have i n i t i a t e d s a l t f lowage. Perhaps t h i s t h e o r y e x p l a i n s t h e f a c t t h a t t h e f o l d s p lunge west, away from t h e a r c h system, and were caused by s l i p down- d i p i n t o t h e Denver Basin.

111. SEASAT DATA INTERPRETATION

A. C h a r a c t e r i s t i c s and Techniques The Seasat s a t e l l i t e opera ted th rouqh t h e summer o f 1978 a t an a l t i t u d e

o f 800 km. It c a r r i e d a s y n t h e t i c a p e r t i r e rada r (SAR) t h a t was a b l e t o image 100-km-wide swaths across b o t h l a n d and sea. The 108" o r b i t a l i n c l i n a t i o n r e s u l t e d i n two ' l o o k ' d i r e c t i o n s f o r t h e imagery, one t r e n d i n g n o r t h e a s t and t h e o t h e r nor thwest . I n t h i s manner, c e r t a i n areas were imaged w i t h two d i f - f e r e n t i l l u m i n a t i o n d i r e c t i o n s . The i n c i d e n c e ang le o f t h e rada r beam a t 20" from t h e v e r t i c a l was n o t optimum f o r g e o l o g i c purposes as i t leads t o con- s i d e r a b l e d i s t o r t i o n i n t o p o g r a p h i c a l l y rugged areas. T h i s was, however, n o t a problem i n t h e g e n t l e topography o f t h e area. A d e s c r i p t i o n o f t h e Seasat imaging rada r and d i s c u s s i o n s o f appl i c a t i o n s t o geology and oceanography can be found i n E l a c h i (1980) .

The Seasat SAR system was a c t i v e , meaning t h a t i t t r a n s m i t t e d and re - ce i ved i t s own energy t o o b t a i n t h e image. Thus, Seasat was independent o f such v a r i a b l e s as sun ang le , t i m e o f day, and weather c o n d i t i o n s . I n con- t r a s t , t h e Landsat s a t e l l i t e i s a pass i ve system, dependent upon these e n v i r o n - mental and p h y s i c a l v a r i a b l e s ( L i l lesand and K i e f e r 1979).

Fac to rs t h a t modulate image tone o r b r i g h t n e s s i n a rada r image a re su r - f a c e s lope and roughness, t h e s c a l e o f t h e r a d a r o p e r a t i n g wavelength, and sur - f ace d i e l e c t r i c cons tan t . The Seasat system, w i t h an i n c i d e n c e ang le o f Z O O ,

i s s e n s i t i v e t o s lopes near t h i s ang le which face t h e radar . A t t h i s wave- l e n g t h , low f a u l t scarps a re b r i g h t specu la r r e f l e c t o r s ; and s u r f a c e roughness causes cons ide rab le energy t o be backsca t te red , r e s u l t i n g i n a b r i g h t image tone.

P l a t e 3 ( i n s i d e back cove r ) i s a Seasat image o f t h e s tudy area i n western Nebraska. P l a t e 4 ( i n s i d e back cove r ) shows l i neamen ts mapped from t h e image. These l ineaments were chosen acco rd ing t o seve ra l c r i t e r i a . L i n e a r , b r i g h t 1 i n e s w h i c h may r e p r e s e n t s t r e a m c h a n n e l s , s c a r p s , and

7

1 i t h o l o g i c boundar ies were mapped. Tonal v a r i a t i o n s t h a t suggested e i t h e r a change i n s u r f a c e l i t h o l o g y o r perhaps a f a u l t were recorded. The chances o f a s u r f i c i a l l i t h o l o g i c boundary d i m i n i s h as t h e l i n e a r i t y o f t h e f e a t u r e i nc reases , suggest ing f a u l t i n g and/or f r a c t u r i n g . F i n a l l y , a 1 i n e t h a t shows d i s c o n t i n u i t i e s t o e i t h e r s i d e s t r o n g l y suggests t h a t t h e r e has been some o f f s e t . The l a s t two c r i t e r i a a re e s p e c i a l l y i n d i c a t i v e o f f a u l t i n g , whereas t h e f i r s t may rep resen t j o i n t s e t s t h a t o f t e n appear c l e a r l y on s a t e l l i t e imagery.

P o s s i b l e f o l d i n g shown on P l a t e 4 was i n f e r r e d on t h e b a s i s o f t o n a l v a r i - a t i o n s on t h e imagery. I n t e r p r e t a t i o n o f f o l d i n g i s d i f f i c u l t i n t h i s a rea , because n e a r l y f l a t - l y i ng T e r t i a r y sediments l y i n g unconformably above t h e f o l d e d s e c t i o n f r e q u e n t l y mask t h e o l d e r g e o l o g i c s t r u c t u r e .

B. Trends on Seasat Imagery P l a t e 4 shows t h e r e s u l t s o f i n t e r p r e t a t i o n based on Seasat imagery.

South o f N40° l a t i t u d e p o s s i b l e broad, g e n t l e f o l d s a re shown, t r e n d i n g e a s t - west ( P l a t e 4 ) . Lineaments do n o t show up w e l l i n t h i s area due t o t h e poor c o n t r a s t o f t h e image and due t o m o d i f i c a t i o n o f t h e s u r f a c e by fa rm ing a c t i v i t y . A s t r o n g nor thwest t r e n d i s de tec ted t o t h e n o r t h o f N40° l a t i t u d e ; i n f a c t , t h i s i s t h e dominant t r e n d th roughou t t h e n o r t h e r n s e c t i o n o f t h e map as i n d i c a t e d by l ineaments v i s i b l e on Seasat imagery. Fur thermore, t h i s t r e n d suggests a r e l a t i o n o f t h e sur face landforms t o t h e Cambridge arch. I n t h i s area topograph ic r e l i e f i s g r e a t e r t han t o t h e south; as a r e s u l t , a g r e a t e r number o f s t r u c t u r a l f e a t u r e s a re expressed on t h e imagery.

The Siouxana a rch cannot be d e l i n e a t e d w i t h Seasat imagery ( P l a t e 4 ) . A p o o r l y d e f i n e d group o f l ineaments , v i s i b l e i n t h e area o f N41° l a t i t u d e , W l O O O l o n g i t u d e , may be r e l a t e d t o t h e Siouxana. The Siouxana a rch has n o t been as a c t i v e as t h e Cambridge system; t h i s i n a c t i v i t y may account f o r t h e l a c k o f w e l l - d e f i n e d l ineaments . S t a n l e y and Wayne (1972) , on t h e b a s i s o f m in ima l P l e i s t o c e n e movement o f t h e Siouxana, a l s o b e l i e v e t h a t t h e r e has been 1 i t t l e recen t a c t i v i t y t he re .

Another subord ina te s e t o f east -west l i n e a r t r e n d s i s v i s i b l e among t h e dune f i e l d s o f t h e M i d d l e Loup R iver . It i s p o s s i b l e t h a t a e o l i a n a c t i o n has c r e a t e d these l ineaments , b u t s t r u c t u r a l r e l i e f may be v i s i b l e th rough t h e dune d e p o s i t s and may r e l a t e t o t h e movement over s a l t d e p o s i t s p resen t t o t h e west o f W102O l o n g i t u d e .

Frequency-azimuth h is tograms were drawn f rom 1 ineaments on P1 a t e 4 ( F i g . 3 ) . O v e r a l l , a s t r o n g no r th -no r thwes t t r e n d i s observed w i t h a mean az imuth o f 335" f o r a l l l i neaments ( F i g . 3a). T h i s t r e n d i s s u b p a r a l l e l t o t h e r e g i o n a l no r thwes t s t r i k e o f t h e Cambridge arch. However, i n t h i s area t h e a r c h t r e n d s i r r e g u l a r l y no r th -no r thwes t when mapped f rom e x p l o r a t i o n d r i l l h o l e s t h a t p e n e t r a t e t h e Precambrian s u r f a c e ( P l a t e 2 ) (Car l son 1966).

The e f f e c t of p e r i o d i c u p l i f t o f t h e Cambridge a rch on l ineament o r i e n - t a t i o n s can be seen when comparing d i f f e r e n t areas. Between N41" - N42" l a t i t u d e , W100" - W101" l o n g i t u d e , l i neamen ts t r e n d a t a mean az imuth o f 321" (F ig . 3b) . T h i s number c o r r e l a t e s w e l l w i t h an average t r e n d o f 322" o f t h e a r c h i n t h i s area. A d d i t i o n a l l y , F ig . 3b suggests a bimodal g roup ing o f azimuths. A s t r o n g secondary t r e n d s t r i k i n g 270" - 280" may r e f l e c t t h e bending of t h e a rch t o t h e west near t h e M idd le Loup R i v e r on t h e n o r t h e r n p a r t o f P l a t e 2.

Between N40" - N41" l a t i t u d e , W101" - W102" l o n g i t u d e , l ineaments t r e n d aga in t o t h e n o r t h e a s t ( F i g . 3c) . Removed f rom t h e l o c a l l y f l u c t u a t i n g s t r i k e o f t h e arch, t h e l ineaments assume t h e r e g i o n a l nor thwest t r e n d o f t h e arch.

8

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F i g . 3. Histograms o f o r i e n t a t i o n s o f l ineaments mapped on P l a t e 4. F iq . 3a r e p r e s e n t s o r i e n t a t i o n s of a l l l ineaments. F i g . 3h , 3c, and 3 d rep resen t o r i e n t a t i o n s of l i neamen ts mapped i n i n d i v i d u a l l a t i -

u3 t u d i na l and 1 ong i t u d i na l quadrants.

Between N40" - N41" l a t i t u d e , W100" - W101" l o n g i t u d e , l i neamen ts t r e n d a t a mean az imuth o f 352" ( F i g . 3d) . T h i s more n o r t h e r l y o r i e n t a t i o n i s r e f l e c t e d t o a l i m i t e d degree by t h e n o r t h - n o r t h e a s t t r e n d o f t h e Cambridge a r c h i n t h i s area. These l i neamen ts s t r i k e o b l i q u e l y t o t h e a r c h b u t may p a r a l l e l t h e a r c h as one moves eas t o f t h e s t u d y area.

Thus, if t h e s e l i n e a m e n t s a r e i n d i c a t i o n s o f s u b s u r f a c e s t r u c t u r e ( f a u l t i n g and f r a c t u r i n g ) , t h e s t r i k e o f t h e Cambridge a rch c o n t r o l s t h e t r e n d o f t h e l i neamen ts on t h e c r e s t and f l a n k s o f t h e arch.

A d e t a i l e d comparison of mapped l i neamen ts t o known s t r u c t u r e i s d i s - cussed i n t h e Appendix.

I V . GEOTHERMAL CORRELATIONS

A. Ho t Dry Rock Geothermal Resource G r a n i t i c rocks shown on P l a t e 1 were i n t r u d e d l a t e r t han t h e m a j o r i t y o f

basement rocks i n t h e area (Merr iam and A t k i n s o n 1955; L i d i a k 1969, 1972). Thus, t hese a r e p l u t o n i c rocks younger than 1800 Myr b.p. and may have h o t d r y rock (HDR) geothermal p o t e n t i a l because o f t h e i r composi t ion, age, and r a d i o - gen ic heat p r o d u c t i o n .

The o n l y i n d i c a t i o n o f h o t g r a n i t e s i n Nebraska i s i n t h e extreme n o r t h - west where h i g h hea t f l o w and geothermal g r a d i e n t v a l u e s (112 rnWm-', 66"C/krn) (Gosnold 1981b) suggest t h a t t h e g r a n i t i c basement produces r a d i o g e n i c hea t ( P l a t e 1). The g r a n i t e s i n t h e no r thwes t a r e dated a t 1710 k320 t o 1720 2130 Myr b.p. ( G o l d i c h e t a l . 1966; L i d i a k 1969; 1972). There a r e no hea t f l o w d a t a a s s o c i a t e d w i t h t h e g r a n i t e s i n t h e r e g i o n covered by Seasat ( P l a t e 1). However, t hese rocks a r e dated a t 1510 270 t o 1580 2230 Myr b.p. and a r e t h u s younger t h a n t h e g r a n i t e s i n t h e n o r t h w e s t ( G o l d i c h e t a l . 1966; L i d i a k 1969; 1972). The younger age may be an i n d i c a t i o n o f h i g h e r r a d i o g e n i c heat p ro - d u c t i o n t h a n t h e no r thwes te rn g r a n i t e s .

I t i s i m p o r t a n t f o r HDR e x p l o r a t i o n t o d e l i n e a t e areas t h a t have l i t t l e f a u l t i n g and f r a c t u r i n g i n o r d e r t o m i n i m i z e f l u i d l o s s i n t h e r e s e r v o i r rock . Thus, g r a n i t e s A and E ( P l a t e 1) may be a good t a r g e t f o r HDR i n v e s t i g a t i o n because o f a l a c k o f c a t a c l a s i s and mapped Seasat l i neamen ts i n t h i s r e g i o n , a l t h o u g h l i neamen t 1 may r e p r e s e n t a c o n t a c t between g r a n i t e E and t h e u n d i v i d e d P r e c a m b r i a n rock. The l a c k o f 1 ineaments suggests min ima l f a u l t i n g and/or f r a c t u r i n g i n t h i s area. I n c o n t r a s t , g r a n i t e s B y C y and D appear t o have been e x t e n s i v e l y f r a c t u r e d and f a u l t e d a c c o r d i n g t o t h e 1 ineament ana ly - s i s shown on P l a t e 1 and P l a t e 4. T h i s f a c t may p r e c l u d e these areas f o r HDR e x p l o r a t i o n because o f p o t e n t i a l l y e x t e n s i v e f l u i d l o s s i n t h e g r a n i t i c r e s e r - v o i r rocks .

Another i m p o r t a n t c o n s i d e r a t i o n f o r HDR development i s t h a t a r o c k tem- p e r a t u r e o f 200°C can be reached w i t h i n reasonable d r i l l i n g depths. U n f o r - t u n a t e l y , t h e r e i s no hea t f l o w i n f o r m a t i o n f o r g r a n i t e s A and E. However, i f one p o s t u l a t e s a geothermal g r a d i e n t o f 50"C/km, a tempera tu re o f 200°C can be reached by d r i l l i n g 4 km. F o r t h e west l i m b o f g r a n i t e A, one would have t o d r i l l t h r o u g h 1.4 km o f m a i n l y low- thermal c o n d u c t i v i t y Phanerozoic sha les ( h i g h geothermal g r a d i e n t ) and 2.6 km of Precambrian g r a n i t i c rock t o reach t h e 4-km depth and 200°C tempera tu re ( C a r l s o n 1966). For t h e e a s t l i m b o f g r a n i t e A, t h e d r i l l i n g would p e n e t r a t e 1.2 km o f sediments and 2.8 km o f g r a n i t e . S i m i l a r l y , g r a n i t e E would r e q u i r e d r i l l i n g 1.2 km o f sediments and 2.8 km of g r a n i t e . These f i g u r e s f a l l w i t h i n economic d r i l l i n g depths and c a p a b i l i t i e s .

10

F o r HDR p r o d u c t i o n t o be f e a s i b l e , t h e g r a n i t i c basement r e s e r v o i r rock must have low p e r m e a b i l i t i e s . However, t hese requ i rements have been met i n g r a n i t i c rocks a t Fenton H i l l , New Mexico, t h e i n i t i a l HDR t e s t s i t e . Pre- sumably, g r a n i t e s A and E i n Nebraska c o u l d a l s o meet these requ i rements .

F i n a l l y , t h e p o t e n t i a l HDR s i t e shou ld be l o c a t e d near a p o p u l a t i o n cen te r . G r a n i t e s A and E are ad jacen t t o Nor th P l a t t e , a c i t y of 24 500 peopl e. It? f a c t , g r a n i t e A l i e s j u s t 10 km t o t h e west o f Nor th P l a t t e ( P l a t e 1 ) .

B . Hydrothermal Resource Pub l i shed heat f l o w and geothermal g r a d i e n t da ta do n o t i n d i c a t e h i g h l y

anomalous zones o f heat f l o w i n southwest Nebraska where t h e r e i s Seasat coverage ( P l a t e 1 ) (Gosnold 1981a, 1981b). Thus, t h e f r a c t u r e s and f a u l t s d e l i n e a t e d by Seasat i n t h i s area p robab ly do n o t t r a n s p o r t h o t f l u i d s ; t h e hydrothermal p o t e n t i a l i n t h i s area appears t o be low. However, f u r t h e r heat f low s t u d i e s a re needed t o c o n f i r m t h i s hypothes is .

Yet , such a Seasat s t r u c t u r a l s tudy i n t h e nor thwest and i n t h e panhandle o f Nebraska may prove ex t remely u s e f u l . The h i g h r e g i o n a l heat f l o w and g r a d i e n t s i n t h i s area a re due p r i m a r i l y t o upd ip o f t h e sedimentary s e c t i o n f rom t h e Denver-Ju lesberg bas in eastward t o t h e Chadron-Cambridge arch. Hot water i s c a r r i e d upward by t h e Cretaceous Dakota a q u i f e r a t a r a t e o f 1 m/yr, and geothermal g r a d i e n t s a re enhanced by low thermal c o n d u c t i v i t y sha les i n t h e Phanerozoic s e c t i o n (Gosnold 1981a, 1981b). A Seasat 1 ineament s tudy a long t h e Chadron a rch would d e l i n e a t e zones o f t e n s i o n a l f r a c t u r i n g and f a u l t i n g caused by p e r i o d i c u p l i f t o f t h e arch. Such s t r u c t u r e s have been suggested a t t h e c r e s t o f t h e C i n c i n n a t i - F i n d l e y a rch i n western Ohio (Cannon e t a l . 1980). These f a u l t s and f r a c t u r e s rep resen t l i n e s o f weakness th rough which h o t f l u i d s may c i r c u l a t e beyond t h e Dakota a q u i f e r . Thus, Seasat l i n e a - ments mapped a long t h e s t r u c t u r a l h i g h o f t h e Chadron a r c h would i n d i c a t e zones o f h o t water c i r c u l a t i o n a t r e l a t i v e l y sha l l ow depths as t h e sediments d i p up eastward toward t h e arch.

V. CONCLUSION

It has been shown t h a t Seasat imagery does c o n t r i b u t e t o d e l i n e a t i o n o f s u b t l e s t r u c t u r e s d i f f i c u l t t o d e f i n e i n any way o t h e r than th rough d r i l l i n g . The imagery i s u s e f u l as a p r e l i m i n a r y e x p l o r a t i o n t o o l i n areas o f l i t t l e phys iog raph ic r e l i e f , such as t h e m i d - c o n t i n e n t o f t h e U.S., and can be used f o r reconnaissance o f l a r g e areas. It can be used t o l o c a t e s u r f a c e expres- s ions o f basement s t r u c t u r e and may a i d i n p i n p o i n t i n g s p e c i f i c areas f o r f u r t h e r study. I n combina t ion w i t h o t h e r g e o l o g i c and geophys ica l da ta t h e imagery can be a v a l u a b l e e x p l o r a t i o n t o o l .

ACKNOWLEDGMENTS

I thank W i l l Gosnold, U n i v e r s i t y o f Nebraska, Frank Osterwald, U.S. Geo- l o g i c a l Survey, and A1 S c h i s l e r , Golden, Colorado, f o r t a k i n g t h e t i m e t o t a l k t o me about t h e t e c t o n i c s o f wes tern Nebraska. Grant Heiken o f t h e Los Alamos N a t i o n a l L a b o r a t o r y p rov ided encouragement th roughou t t h e p r o j e c t , espec ia l l y so i n t h e beginning. Thorough rev iews by W i l l Gosnold, Ron Blom, J e t P ropu l - s i o n Labora to ry , and F rase r G o f f , Los A1 amos N a t i o n a l Labora to ry , a re g r e a t l y apprec ia ted .

11

APPENDIX

COMPARISON WITH KNOWN STRUCTURE

Some ve ry i n t e r e s t i n g r e l a t i o n s may be observed by comparing P l a t e s 2 and 4. A n t i c l i n e s a and c and s y n c l i n e b on P l a t e 2 seem t o c o r r e l a t e w i t h l i n e a - ments _ - a, b y a n r c o n - P l a t e 4. The l i n e a m e n t s may r e f l e c t t h e s t r i k e o f bed- d i n g o r may rep resen t j o i n t i n g p a r a l l e l t o t h e axes o f t h e f o l d s . B i l l i n g s (1972) s t a t e s t h a t t h i s k i n d of j o i n t i n g i s o f t e n observed. F. W. Os terwa ld (U.S. Geo log ica l Survey, Golden, Colorado, persona l communication 1980) a1 S O b e l i e v e s t h a t j o i n t s a re seen e a s i l y on s a t e l l i t e imagery. I n c o n t r a s t , syn- c l i n e s d and f and a n t i c l i n e e do n o t l i e on a l i n e w i t h l i neamen ts d, e, and - f. These l ineaments do n o t T e f l e c t s t r i k e o f t h e beds and a r e l e s s T i k e l y j o i n t s ; f a u l t i n g o r f r a c t u r i n g i s p o s s i b l e here. Fur thermore, l i neamen t g corresponds w e l l w i t h f a u l t g; t h e f a u l t zone i s v i s i b l e on t h e Seasat image, To t h e south o f l i neamen t 7 l i e seve ra l l ineaments on t h e same t r e n d . If these l ineaments r e f l e c t f auT ts , t h e y may i n d i c a t e a f a u l t system 30 km wide. The number o f l i neamen ts and t h e e x t e n t o f f a u l t i n g i n d i c a t e d on P l a t e s 2 and 4 demonstrate t h a t t h i s f a u l t zone i s a ma jor basement s t r u c t u r e . L i kew ise , an e x c e l l e n t c o r r e l a t i o n e x i s t s between l i neamen t h and f a u l t h. Again, t h i s s t r u c t u r e i s p robab ly a Precambrian f e a t u r e ; t h i s T& a l o n g f a u T t t h a t extends southeas t t o an area o f s teep d i p and f u r t h e r beyond t o another basement f a u l t a long t h e same t r e n d o f f t h e s tudy area (Co le 1 9 6 2 ) . Lineament h t h a t sp lays o f f f rom l i neamen t h and l i neamen ts h and h a re a l s o p a r t of-?he l i neamen t system and most l i k d y p a r t o f t h e f a d ? s y s t d . These f o u r l i n e a r s r e i n f o r c e t h e t h e o r y t h a t f a u l t h i s a ma jo r basement f e a t u r e . To t h e n o r t h , l i neamen ts a l s o r e f l e c t a s i g n i f i c a n t t rend ; t h e e a s t e r n p a r t o f t h e P l a t t e R i v e r on t h e s tudy area i s s t r u c t u r a l l y c o n t r o l l e d f o r 30 km as i t p a r a l l e l s t h e s t r i k e o f t h e Cambridge arch. To t h e eas t o f f t h e map, t h e c r e s t o f t h e a r c h swings south as t h e r i v e r f lows eas t . Severa l l ineaments , Ll and L2, correspond t o t h i s t rend . The l ineaments may rep resen t t h e f u r t h e s t e x t e n t o f t h e changing course o f t h e r i v e r , b u t t h e s t r i k i n g l i n e a r i t y o f t hese f e a t u r e s suggests t h a t t h e r i v e r course i s s t r u c t u r a l l y c o n t r o l l e d . Moreover, t hese l ineaments may r e f l e c t some shear ing , as l i neamen t -i3 o f f s e t s L1 and i2. Perhaps t h e r e has been r i g h t - l a t e r a l s t r i k e - s l i p movement i n t h i s area. S h e a r i n g a l s o i s i n d i c a t e d where l i neamen t g and l ineaments t o t h e southwest fo rm a c h a r a c t e r - i s t i c ' N U ' p a t t e r n (PeterSon 1979) , i n d i c a t i n g l e f t - l a t e r a l shear ing . Other 'Nu' p a t t e r n s a r e seen t o t h e n o r t h o f t hese l ineaments .

F u r t h e r ev idence f o r s t r u c t u r a l c o n t r o l o f d ra inage i s shown i n t h e M i d d l e Loup R i v e r system. Lineaments j , j , and assoc ia ted l ineaments r e s u l t from an east -west a l ignment o f sand ddes -zea r t h e edges o f t h e r i v e r . Yet, such a l ignments do n o t correspond s o l e l y t o wind a c t i o n , as Russe l l (1929) sup- posed. Two l i n e s o f ev idence suggest basement c o n t r o l o f t hese l ineaments. The f i r s t i s a p a r a l l e l i s m o f t h e l i neamen ts t o s teep aeromagnetic g r a d i e n t j , a l t h o u g h a t a s l i g h t ang le t o aeromagnetic g r a d i e n t i, suggest ing f a u l t i n g . Indeed, t h e r e i s a s t r i k i n g a l ignment o f g r a d i e n t j a d l i neamen ts Jl and J2. Second, t h e western edge of P l a t e 2 shows t h a t t h e M i d d l e Loup R i v e r f o l l o w s t h e c r e s t o f t h e Cambridge arch. It i s s t r o n g l y suspected t h a t t h e r i v e r and dunal p a t t e r n s a re c o n t r o l l e d by basement s t r u c t u r e and t h a t t hese 1 ineaments r e f l e c t more than s u r f i c i a l processes. Very p o s s i b l y basement f a u l t i n g i s showing th rough t h e a e o l i a n depos i t s .

12

13

T h i s east-west t r e n d p a r a l l e 1 s t h e west -p l ung ing f o l d s t o t h e southwest t h a t p robab ly a r e c o n t r o l l e d by s a l t t e c t o n i c s . Yet, t h e ev idence i n d i c a t e s t h a t s t r u c t u r a l c o n t r o l by t h e a r c h i s dominant near t h e M i d d l e Loup R i v e r .

No tab ly , t h e wes t -p lung ing f o l d s $ , k ~ 2 , and k3 sou th o f t h e South P l a t t e R i v e r s t r i k e o b l i q u e l y t o l i neamen ts k , T h i s suggests these l i n e - aments a r e n o t r e l a t e d s o l e l y t o saTl! t 2 Z t o n i c s . I n f a c t , t h e i n f l u e n c e o f t h e a r c h seems t o be dominant ove r s imp le s a l t movement. Perhaps t h e two proc- esses combine t o c r e a t e t e c t o n i c f e a t u r e s ; f o r example, l i neamen t has a s t r o n g e r east -west t r e n d t h a t p a r a l l e 1 s sync1 i n e k

Yet, i n t h e area o f t h e arch, c o n t r o l o f s t r u c t u r e and t e c t o n i c s by t h e a r c h dominates, w i t h p e r i o d i c u p l i f t caus ing de fo rma t ion . Coupled w i t h s a l t t e c t o n i c s r e s u l t i n g f rom t h e d e p o s i t i o n o f Permian e v a p o r i t e s , t h e amount o f deformat ion i nc reases d ramat i ca l l y .

, and L3.

-4.

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14

Osterwal d , F. W. cl B. G. De , "P e l i m i n a r y Tec ton ic Map o f We t e r n Nebraska and Nor thwestern Kansas Showing t h e D i s t r i b u t i o n o f Uranium Depos i ts ,I' U.S. Geo log ica l Survey Map MF 129, sca le 1:500 000 (1958) .

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1135-1150 (1958) .

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PLATE I PRECAMBRIAN LITHOLOGY, GEOTHERMALAND RADIOMETRIC AGE CORRELATIONS, NEBRASKA

9

I 102" Measured geothermal gradient in OC/Km with heat flow in parentheses in mWm-*

Rb-Sr radiometric age in millions of years

Seasat SAR satellite coverage

Letters refer to text

I 101O

South Dakota

9 62 (68)

I 1 ooo

I I I I I

I I I I I

10 0 10 20 30 40

10 0 10 20 30 40 50 1 Miles

I Kilometers

Sources of data: Carlson, 1966 Gosnold, Lidiak, 1972 1981 b

\

PLATE 2 KNOWN TECTONIC FEATURES, WESTERN NEBRASKA

102"'

PLATE 3 SEASAT SAR SATELLITE IMAGE,

WESTERN NEBRASKA 390 400

II 40 Miles 2 0 30 ' 0 ~

10 5 0 3

1 0 5 0 1 0 20 30 40 50 Kilometes I- -

1020

11000

Zietz ef a/ , 1966, Plate 2

r - - 77- 7 EXPLANATION

i-, / ,/ Nebraska 'I I D NUMBER: 109 I ' REVOLUTION: 559

Colorado STATION: GDS 1 DATE: 12-28-78 Kansas I

L_- -1 START TIME: 02:42:31 Location Map DURATION: 8:30

4 ? O 101O

1020'

420 \

390 1 070 40°

PLATE 4 STRUCTURAL INTERPRETATION SASED ON SEASAT EXPLANATION

SATELLITE IMAGERY, WESTERN NEBRASKA - Strong Lineament L -./'I Possible Folding

-- 4- Weak Lineament h 4 Letters refer to text

390 40° 1020 41" 4:O 1010